Chapter 18: Nutrition and Metabolism

Appetite is the drive that compels us to seek food. Several types of interacting hormones control appetite by

affecting part of the hypothalamus called the arcuate nucleus

Proteins are composed of

amino acids. They have a wide variety of functions.

The amount of oxygen the body consumes is directly proportional to the

amount of energy released by cellular respiration. The BMR indicates the total amount of energy expended in a given time to support the activities of such organs as the brain, heart, lungs, liver, and kidneys.

Today a measurement termed body mass index (BMI) is used to

assess weight considering height, and has become the basis of classifying a person as underweight, normal weight, overweight, or obese

An infant also needs to gain weight rapidly,

best accomplished by drinking human milk, which has more total carbohydrate than prepared formulas. The high fat content of human milk is important for the rapid growth of the infant's brain, where many neurons are ensheathed in lipids.

The liver uses cholesterol to produce

bile salts. Cholesterol is not used as an energy source, but it does provide structural material for cell and organelle membranes, and it furnishes starting materials for the synthesis of certain sex hormones and hormones produced by the adrenal cortex.

Certain kidney disorders impair the removal of urea from the blood, raising the blood urea concentration. A blood test called

blood urea nitrogen (BUN) determines the blood urea concentration and is often used to evaluate kidney function

A state of energy balance exists when

caloric intake in the form of foods equals caloric output from the basal metabolic rate and muscular activities. Under these conditions, body weight remains constant, except perhaps for slight variations due to changes in water content.

The most obvious and common nutritional disorders reflect

calorie imbalances, which may result from societal and geographic factors.

The amount of potential energy a food contains is expressed as

calories (kal′o-rēz), which are units of heat.

To maintain weight,

calories in must equal calories expended.

The protein constituents of lipoproteins in the outer layer, called apoproteins or apolipoproteins,

can combine with receptors on the membranes of specific target cells. Lipoprotein molecules vary in the proportions of the lipids they contain.

Saturated fats in excess are a risk factor for

cardiovascular disease.

Monounsaturated fats (such as from avocado and olives) promote

cardiovascular health, whereas saturated fats (such as those in butter or lard) raise the risk of heart disease.

Low-density lipoproteins (LDL) have a high concentration of

cholesterol and are the major cholesterol-carrying lipoproteins.

Many plant proteins have too little of one or more essential amino acids to provide adequate nutrition for a person. However,

combining appropriate plant foods can supply an adequate diversity of dietary amino acids. For example, beans are low in methionine but have enough lysine. Rice lacks lysine but has enough methionine. A meal of beans and rice offers enough of both types of amino acids.

Glucose is the carbohydrate most

commonly oxidized in glycolysis for cellular fuel.

Proteins are classified as

complete or incomplete based on the amino acids they provide.

Good sources of linoleic acid include

corn oil, cottonseed oil, and soy oil. Linolenic acid is another essential fatty acid.

Lipoproteins are classified on the basis of their

densities, which reflect their composition.

When a person needs to gain weight,

diet can be altered to include more calories and to emphasize particular macronutrients. For example, a person recovering from a debilitating illness might consume more carbohydrates, whereas a bodybuilder might eat extra protein to hasten muscle development.

The simple carbohydrates include

disaccharides ("double sugars") from milk sugar, cane sugar, beet sugar, and molasses and monosaccharides ("single sugars") from honey and fruits.

The human body can synthesize many amino acids (nonessential amino acids). However,

eight amino acids the adult body needs (ten required for growing children) cannot be synthesized sufficiently or at all, and they are called essential amino acids. This term refers only to dietary intake, because all amino acids are required for normal protein synthesis.

Maintenance of body temperature may require additional energy expenditure, particularly in cold weather. In this case,

extra energy is expended in involuntary muscular contractions, such as shivering, or through voluntary muscular actions, such as walking. Growing children and pregnant women, because their bodies are actively producing new tissues, also require more calories.

Complete proteins

(also called high-quality proteins) have adequate amounts of all of the essential amino acids to maintain human body tissues and promote normal growth and development. Certain proteins in milk, meat, and eggs are complete.

Incomplete proteins

(also called low-quality proteins) lack one or more of the essential amino acids. For example, zein in corn has too little of the essential amino acids tryptophan and lysine. Zein is unable by itself to maintain human tissues or to support normal growth and development.

For an average adult, nutritionists recommend a daily protein intake of about

0.8 gram per kilogram (0.4 gram per pound) of body weight or 10% of a person's diet. Another way to estimate desirable protein intake is to divide weight in pounds by 2.

Although a calorie is defined as the amount of heat required to raise the temperature of a gram of water by 1 degree Celsius (°C), the calorie used to measure food energy is

1,000 times greater. This large calorie (Cal.) equals the amount of heat required to raise the temperature of a kilogram (1,000 grams) of water by 1°C (from 14.5°C to 15.5°C) and is also equal to 4.184 joules.

Glycerol and fatty acid molecules resulting from the hydrolysis of fats can also combine to form

fat molecules in anabolic reactions and be stored in fat tissue. Additional fat molecules can be synthesized from excess glucose or amino acids.

Unsaturated fats contain

fatty acid molecules with double bonds in their carbon chains and are in seeds, nuts, and plant oils

Once fatty acid molecules have been activated, other enzymes called

fatty acid oxidases in mitochondria break them down. This phase of the reactions removes successive two-carbon segments of fatty acid chains. In the liver, some of these segments react to produce acetyl coenzyme A molecules. Excess acetyl CoA molecules can be converted into compounds called ketone bodies, such as acetone, which later may be changed back to acetyl coenzyme A. In either case, the resulting acetyl coenzyme A can be oxidized in the citric acid cycle. The glycerol parts of the triglyceride molecules can also enter metabolic pathways leading to the citric acid cycle, or they can be used to synthesize glucose.

In the first phase of beta oxidation,

fatty acids are activated. This change requires energy from ATP and a special group of enzymes called thiokinases. Each of these enzymes can act upon a fatty acid that has a particular carbon chain length.

More than twice as much energy is derived from equal amounts by weight of fats as from either proteins or carbohydrates. This is one reason why avoiding

fatty foods helps weight loss, if intake of other nutrients does not substantially increase. Fats encourage weight gain because they add flavor to food, which can cause overeating. However, fatty foods satisfy hunger longer than carbohydrate-rich foods.

Males typically have higher metabolic rates than

females. As body temperature, blood level of thyroxine, or blood level of epinephrine increase, so does the BMR. The BMR can also increase when the level of physical activity increases during the day.

Sugar substitutes provide concentrated sweetness, so

fewer calories are needed to sweeten a food compared to table sugar (sucrose). Stevia is extracted from leaves of an herb, and is 30 times as sweet as table sugar.

The monosaccharides absorbed from the digestive tract include

fructose, galactose, and glucose. Liver enzymes catalyze reactions that convert fructose and galactose into glucose

All twenty types of amino acids must be in the body at the same time for

growth and tissue repair to occur. In other words, if the diet lacks one essential amino acid, the cells cannot synthesize protein. Essential amino acids are not stored. Those not used to make proteins are oxidized as energy sources or are converted into carbohydrates or fats.

The amounts and types of fats required for health vary with individuals'

habits, goals, and stage of life. For example, infant formula contains two long-chain polyunsaturated fatty acids (docosahexaenoic acid and arachidonic acid) that are in breast milk and are vital for development of the infant's nervous system, particularly the eyes.

The site of the double bond that contributes to a fat's degree of unsaturation is also important. Omega-3 fatty acids, which have double bonds between the third and fourth carbons, are more

healthful than omega-6 fatty acids, with double bonds between the sixth and seventh carbons. Omega-3 fatty acids are found in fish; omega-6 fatty acids are in red meat.

Monounsaturated fats (in which fatty acids contain one double bond), such as those in olive, peanut, and canola oils, are the

healthiest

The types and locations of the chemical bonds between carbon atoms of fatty acid molecules affect

how healthful the fat is.

Before a triglyceride molecule can release energy, it must undergo

hydrolysis. This happens when digestion breaks triglycerides down into fatty acids and glycerol. After being absorbed, these products are carried by the lymph to the blood, then on to tissues. As figure 18.3 shows, some of the resulting fatty acid portions can then form molecules of acetyl coenzyme A (acetyl CoA) by a series of reactions called beta oxidation, which occurs in the mitochondria.

If the blood lipid concentration drops (in response to fasting, for example), some of these stored triglycerides are

hydrolyzed into free fatty acids and glycerol and are released into the bloodstream.

Carbohydrates, fats, and proteins supply energy, which is required for all metabolic processes and therefore

important to cell survival. If the diet is deficient in energy-supplying nutrients, structural molecules may gradually be consumed, leading to death. On the other hand, excess intake of energy-supplying nutrients may lead to obesity, which also threatens health.

Carbohydrates provide the primary source of fuel for cellular processes. The need for carbohydrates varies with

individual energy requirements.

A joule is the

international unit of heat and energy. A large calorie is also called a kilocalorie, but it is customary in nutritional studies to refer to it as a calorie.

Cholesterol is abundant in

liver and egg yolk and, to a lesser extent, in whole milk, butter, cheese, and meats. Foods of plant origin do not contain cholesterol.

Foods rich in proteins include

meats, seafood, poultry, cheese, nuts, milk, eggs, and cereals. Legumes, including beans and peas, contain less protein.

Caloric values determined in a bomb calorimeter are somewhat higher than the amount of energy that

metabolic oxidation releases, because nutrients generally are not completely absorbed from the digestive tract. Also, the body does not completely oxidize amino acids, but excretes parts of them in urea or uses them to synthesize other nitrogenous substances. When such losses are considered, cellular oxidation yields on the average about 4.1 calories from 1 gram of carbohydrate, 4.1 calories from 1 gram of protein, and 9.5 calories from 1 gram of fat.

Digestion ultimately breaks carbohydrates down to

monosaccharides, which are small enough to be absorbed into the bloodstream.

Obesity is prevalent in nations where food is plentiful and diverse. The tendency to become obese may be a holdover from thousands of years ago, when the ability to store energy in the form of fat was a survival advantage when food supplies were scarce or erratic. Today in many African nations,

natural famines combined with political unrest cause mass starvation.

In addition to supplying essential amino acids, proteins provide

nitrogen and other elements for the synthesis of nonessential amino acids and certain nonprotein nitrogenous substances. The amount of dietary protein individuals require varies according to body size, metabolic rate, and nitrogen balance condition.

Saturated fats contain

no double bonds between the carbons of their fatty acid molecules and are mainly found in foods of animal origin, such as meat, eggs, milk, and lard, as well as in palm and coconut oils. bonds

A person more than 20% above the desired weight or with a BMI over 30, is obese, although

obesity (o-bēs′ĭ-te) is more correctly defined as excess adipose tissue. Therefore, overweight and obesity are not the same.

We eat to

obtain the nutrients that power the activities of life. Eating is a complex, finely tuned homeostatic mechanism that balances nutrient intake with nutrient use. Too few nutrients, and disorders associated with malnutrition result. Too many nutrients, and obesity is the consequence.

High-density lipoproteins (HDL) have a relatively high concentration of

protein and a lower concentration of lipids.

A few hours after a meal,

protein catabolism, through the process of gluconeogenesis, becomes a major source of blood glucose. However, metabolism in most tissues soon shifts away from glucose and toward fat catabolism as a source of ATP. Thus, energy needs are met in a way that spares proteins for tissue building and repair, rather than being broken down and reassembled into carbohydrates to supply energy. Using structural proteins to generate energy causes the tissue-wasting of starvation.

Lipids are less dense than

proteins

Overweight is defined as exceeding desirable weight by

10% to 20%, or a BMI between 25 and 30.

Physically active individuals require more carbohydrates than those who are sedentary. The minimal requirement for carbohydrates in the human diet is unknown, but getting at least

125 to 175 grams daily is probably necessary to avoid protein breakdown and to avoid metabolic disorders resulting from excess fat use. An average diet includes 200 to 300 grams of carbohydrates daily.

Aspartame, a dipeptide, is

200 times as sweet; the artificial sweetener saccharin is 300 times as sweet; and sucralose is 600 times as sweet as sucrose. Sucralose is derived from sucrose, and includes chloride.

The lipid content of human diets varies widely. A person who eats mostly burgers, fries, and shakes may consume

50% or more of total daily calories from fat. For a vegetarian, the percentage may be far lower. The USDA and the American Heart Association recommend that lipid intake not exceed 30% of the total daily calories.

Most people should consume

60-150 grams of protein a day. For a pregnant woman, who needs to maintain a positive nitrogen balance, the recommendation adds 30 grams of protein per day. Similarly, a nursing mother requires an additional 20 grams of protein per day to maintain milk production.

Cellulose

a complex carbohydrate abundant in our food—it provides the crunch to celery and the crispness to lettuce. We cannot digest cellulose, and most of it passes through the alimentary canal largely unchanged. However, cellulose provides bulk (also called fiber or roughage) against which the muscular wall of the digestive system can push, facilitating the movement of intestinal contents. Hemicellulose, pectin, and lignin are other plant carbohydrates that provide fiber.

bomb calorimeter

a device used to measure the caloric contents of foods. It consists of a metal chamber submerged in a known volume of water. A food sample is dried, weighed, and placed in a nonreactive dish inside the chamber. The chamber is filled with oxygen and submerged in the water. Then, the food is ignited and allowed to completely oxidize. Heat released from the food raises the temperature of the surrounding water, and the change in temperature is measured. Because the volume of the water is known, the amount of heat released from the food can be calculated in calories.

Very-low-density lipoproteins (VLDL) have

a high concentration of triglycerides.

The amount of energy required to support metabolic activities for twenty-four hours varies from person to person. The factors that influence individual energy needs include

a measurement called the basal metabolic rate, the degree of muscular activity, body temperature, and rate of growth.

Ketones are released when

a person fasts, forcing body cells to metabolize a large amount of fat. People with diabetes mellitus may develop a serious imbalance in pH called ketoacidosis when acetone and other acidic ketones accumulate.

If caloric intake exceeds expenditure,

a positive energy balance occurs, and tissues store excess nutrients. This increases body weight because excess calories are stored as fat. Conversely, if caloric expenditure exceeds input, the energy balance is negative, and stored materials are mobilized from the tissues for oxidation, causing weight loss.

The basal metabolic rate (ba′sal met″ah-bol′ik rāt), or BMR, measures the

rate at which the body expends energy under basal conditions—when a person is awake and at rest; after an overnight fast; and in a comfortable, controlled environment. Tests of thyroid function can be used to estimate a person's BMR.

The liver can convert fatty acids from one form to another. However,

the liver cannot synthesize certain fatty acids, called essential fatty acids, which must be obtained in the diet. Linoleic acid, for example, is an essential fatty acid required to synthesize phospholipids, which, in turn, are necessary for constructing cell membranes and myelin sheaths, and for transporting circulating lipids.

In addition to regulating circulating lipids,

the liver controls the total amount of cholesterol in the body by synthesizing and releasing it into the blood or by removing cholesterol from the blood and excreting it into the bile.

If foods do not provide an adequate supply of carbohydrates,

the liver may convert some noncarbohydrates, such as amino acids from proteins or glycerol from fats, into glucose—a process called gluconeogenesis. The requirement for glucose has physiological priority over the need to synthesize certain other substances, such as proteins, from available amino acids.

The liver uses free fatty acids to synthesize

triglycerides, phospholipids, and lipoproteins that may then be released into the bloodstream. These lipoproteins are large and consist of a surface layer of phospholipid, cholesterol, and protein surrounding a triglyceride core.

Adipose tissue stores excess

triglycerides.

Most foods containing starch and glycogen have many other nutrients including

valuable vitamins and minerals.

Lipids

organic compounds that include fats, oils, phospholipids, and cholesterol. They supply energy for cellular processes and help build structures, such as cell membranes. The most common dietary lipids are the fats called triglycerides (tri-glis′er-īdz), consisting of three fatty acid molecules and a glycerol molecule.

Carbohydrates

organic compounds that include the sugars and starches. The energy held in their chemical bonds is used to power cellular processes.

The lipids in foods are

phospholipids, cholesterol, and, most commonly, fats (triglycerides). Lipids provide a variety of physiological functions; however, fats mainly supply energy. Gram for gram, fats contain more than twice as much chemical energy as carbohydrates or proteins.

Triglycerides are found in

plant- and animal-based foods.

Carbohydrates are ingested in a variety of forms. Complex carbohydrates include the

polysaccharides ("many sugars"), such as starch from plant foods and glycogen from meats.

Several factors influence food intake, including

smell, taste, and texture of food; neural signals triggered by stretch receptors in the stomach; stress; and hormones.

Many cells can also oxidize fatty acids to obtain energy. However,

some cells, such as neurons, normally require a continuous supply of glucose for survival. (Under some conditions, such as prolonged starvation, other fuel sources may become available for neurons.) Even a temporary decrease in the glucose supply may seriously impair nervous system function. Consequently, the body requires a minimum amount of carbohydrate.

When ketone bodies form faster than they can be decomposed,

some of them are eliminated through the lungs and kidneys. When this happens, the ketone acetone may impart a fruity odor to the breath and urine.

Fat intake must be sufficient throughout life to

support absorption and transport of fat-soluble vitamins. Most adults who eat a variety of foods obtain adequate fats.

A person who is starving has a negative nitrogen balance because

the amount of nitrogen excreted as a result of amino acid oxidation exceeds the amount the diet replaces. Conversely, a growing child, a pregnant woman, or an athlete in training is likely to have a positive nitrogen balance because more protein is being built into new tissue and less is being used for energy or excreted.

Glycogen can be rapidly broken down to yield glucose (glycogenolysis) when it is required to supply energy. However,

the body can store only a certain amount of glycogen, and excess glucose reacts to form fat (lipogenesis), which is stored in adipose tissue

Protein deficiency causes tissue wasting and also decreases the level of plasma proteins, which decreases

the colloid osmotic pressure of the plasma. As a result, fluids collect in the tissues, producing nutritional edema.

If energy is not required immediately,

the deaminated parts of the amino acids may react to form glucose or fat molecules in other metabolic pathways

as the proportion of lipids decreases,

the density increases.

As the proportion of lipids in a lipoprotein increases,

the density of the particle decreases.

A healthy adult continuously builds up and breaks down proteins. Proteins are metabolized at different rates in different tissues, but

the overall gain of body proteins equals the loss, producing a state of dynamic equilibrium. Because proteins have a high percentage of nitrogen, dynamic equilibrium also brings nitrogen balance (ni′tro-jen bal′ans), in which the amount of nitrogen taken in equals the amount excreted.

Several pathways decompose the remaining deaminated parts of amino acids. The specific pathways that are followed depend upon

the particular type of amino acid being dismantled. Some pathways form acetyl coenzyme A, and others more directly lead to steps of the citric acid cycle. Most of the energy released from the cycle is captured in molecules of ATP

When dietary proteins are digested,

the resulting amino acids are absorbed and carrried by the blood to cells. Many of these amino acids are used to form new protein molecules, as specified by DNA base sequences. These new proteins include enzymes that control the rates of metabolic reactions; clotting factors; the keratins of skin and hair; elastin and collagen of connective tissue; plasma proteins that regulate water balance; the muscle components actin and myosin; certain hormones; and the antibodies that protect against infection

Cells also use carbohydrates as starting materials for synthesizing such vital biochemicals as the 5-carbon sugars ribose and deoxyribose, which are

the sugars required for the production of the nucleic acids RNA and DNA. The disaccharide lactose (milk sugar) is synthesized when the breasts are actively secreting milk.

Protein molecules may also supply energy. To do this,

they must first be broken down into amino acids. The amino acids then undergo deamination, a process in the liver that removes the nitrogen-containing amino groups (—NH2), which then react to form a waste called urea (u-re′ah) (see fig. 2.17). The blood carries urea to the kidneys, where it is excreted in urine.

Maintaining the BMR usually requires the body's greatest expenditure of energy. The energy required to support voluntary muscular activity comes next, though

this amount varies greatly with the type of activity (table 18.3). For example, the energy to maintain posture while sitting at a desk might require 100 calories per hour above the basal need, whereas running or swimming might require 500-600 calories per hour.

The BMR for an average adult indicates a requirement for approximately 1 calorie of energy per hour for each kilogram of body weight. However,

this requirement varies with sex, body size, body temperature, and level of endocrine gland activity. For example, because heat loss is directly proportional to the body surface area, and a smaller person has a greater surface area relative to body mass, he or she will have a higher BMR.

It is difficult to determine a desirable body weight. In the past,

weight standards were based on average weights and heights in a certain population, and the degrees of underweight and overweight were expressed as percentage deviations from these averages. These standards reflected the gradual gain in weight as people age. Then medical researchers recognized that such an increase in weight after the age of twenty-five to thirty years is not necessary and may not be healthy. This lead to developing standards of desirable weights.

Some excess glucose is polymerized to form glycogen (glycogenesis),

which the liver and muscles store as a glucose reserve.